For many years, the animal science industry has tried to increase the efficiency of feed utilization in both ruminant and non-ruminant animals. More study has been done in ruminant animals because of their greater economic importance.
In the source of investigating the efficiency of feed use, the mechanism by which ruminants digest and degrade the components of their feed to form molecules which can be metabolically utilized has been intensively studied. Aspects of the mechanism of carbohydrate utilization are now known. Microorganisms in the rumen of the animal ferment carbohydrates to produce monosaccharides, and then degrade the monosaccharides to pyruvate.
Pyruvate is then metabolized by microbiological processes to either acetate or propionate, which may be either acids or derivatives of the acids. Two acetates may be combined thereafter, still in the rumen, to form butyrate. Leng, "Formation and Production of Volatile Fatty Acids in the Rumen." Physiology of Digestion and Metabolism in the Ruminant (Phillipson et al., ed.), Oriel Press, pp. 408-410.
The animal can utilize butyrate, propionate, and acetate with differing degrees of efficiency. (Smith, Gary E. 1971. Energy Metabolism and Metabolism of Volatile Fatty Acids. Digestive Physiology and Nutrition of Ruminants. O.S.U. Book Stores. Corvallis, Oregon). Utilization of these compounds, which are collectively known as volatile fatty acids (VFA), occurs after absorption from the gut of the animal. Butyrate is utilized most efficiently, and acetate the least efficiently. However, the relative efficiency of use of butyrate is negated by the inefficiency of the manufacture of butyrate, which must be made from acetate in the rumen.
The process of formation of acetate in the rumen is one of the major inefficiencies in the rumen. Since acetate is made by the degradation of a pyruvate molecule, each molecule of acetate which is produced is accompanied by a one carbon molecule which subsequently results in the formation of methane. Most of the methane produced is lost through eructation. Since butyrate is made from two molecules of acetate, each molecule of the relatively efficiently used butyrate involves the loss to the animal of two molecules of methane, with all of the associated energy.
Thus, the efficiency of carbohydrate utilization (carbohydrates, being the major nutritive portion of ruminant animals' feed) can be increased by treatments which encourage the animal to produce propionate and butyrate rather than acetate from carbohydrates. Further, the efficiency of feed use can be effectively monitored by observing the production and concentration of propionate compounds in the rumen. If the animal is making more propionates, it will be found to be using its feed more efficiently.
The efficiency of feed utilization by a ruminant animal can readily be determined by chemical analysis for acetate and propionate produced by the fermentation occurring in the rumen or more conveniently by chemical analysis of the rumen contents for acetate and propionate using an in vitro rumen fermentation test.